So joyous and immense were the hopes that once rested on the actor, raconteur and humanitarian Sir Peter Ustinov, who has died in Switzerland aged 82, that the final balance-sheet of his life was bound to seem an anticlimax, both to himself and to those who saw the skyrocket of his early talent.

To his contemporary Richard Attenborough, and to many others, "There was no doubt that he was the genius of our generation. We regarded his potential to be as great as Chekhov or Shaw." But, Attenborough added when Ustinov was nearly 70, "he hasn't yet written what he is capable of - largely due to his diversity of talent".

He never did, and he knew it. He had to be content with the immense joy that he did give, apparently effortlessly; with being the most consistently funny raconteur of his time, recognised as a peer by virtually all other humorists, such as Frank Muir (obituary, January 3 1998), who called him "one of the best-loved people in the world".

From his late teens until old age, with a steadily wider audience, he enriched the gaiety of nations and added to the public stock of harmless pleasure. His global public will be, as Dr Johnson said of David Garrick, "disappointed by that stroke of death" which eclipses his gaiety. His mother was convinced he would be a great creative genius. By the standards he set himself, those of the old European high culture of his mixed ancestry, he fell depressingly short. His genius turned out to be mainly for life-enhancement.

His London stage debut at the age of 18 is still a legend 64 years later. He had West End audiences and the leading theatre critics of the day convulsed with laughter at a brief revue sketch. One of his later party pieces was to imitate every orchestral instrument in Beethoven's Eroica symphony.

His early impact as a comic performer was comparable to that of Peter Cook, John Cleese and Rowan Atkinson; but Ustinov also proved to be a playwright of substance, a film director, a novelist, a heavyweight newspaper columnist and an Academy Award-winning actor. His conviviality and breadth of interests gave him access to most of the world's VIPs. His close friends ranged from Mikhail Gorbachev to Yehudi Menuhin.

He published more than 20 books and harvested countless awards and honorary degrees. His career was like a firework display which never seemed to end. The highest-rising rockets and the great, all-illuminating Roman candles ran out eventually, but the squibs and firecrackers were still entertaining; and the occasional extravagant setpieces remained gorgeous.

He remained haunted by his father's remark about his literary debut: "It is not even drama - it is vaudeville." In the end, he may have fallen below his mother's expectations. But - along with a few others, including Spike Milligan, Tony Hancock and perhaps the early Mel Brooks - he came as close to genius as any humorist in his time. Late in life he kept calling himself a failure, while sounding remarkably comfortable about it.

"There was only one saving grace," he wrote in his autobiography Dear Me (1977), which sold a million copies, "and that was that I was irrevocably betrothed to laughter, the sound of which has always seemed to me the most civilised music in the universe."

His parental heritage was hard to live up to. His father was Jona 'Klop' Ustinov, an opportunistic journalist, half- Russian, half-German, "the best raconteur I have ever met", according to Dame Rebecca West. Until recently Klop, whose nickname meant "Bedbug", was remembered chiefly for running a German news agency in London after the second world war.

But in 1999, a study drawing on unreleased MI5 files disclosed that, as an agent for British intelligence before the war, he warned the government seven months in advance of Hitler's intention to invade Czechoslovakia.

In 1938-39 Klop was press officer at the German embassy in London. The ambassador, Joachim von Ribbentrop, was a confidant of Hitler. Ustinov passed on material not only proving Nazi intent, but showing that Hitler's private remarks about the British prime minister, Neville Chamberlain, were littered with schoolboy obscenities.

Chamberlain's combined anxiety and rage over this "contributed materially" towards his introduction of military call-up early in 1939, according to the study. Klop had made a small difference to history; something that his son never managed.

Peter's mother was the painter and ballet designer Nadia Benois, of Russian, French and Italian blood. Her uncle was the theatrical art director, painter and librettist Alexander Benois, co-founder of the magazine Mir Iskusstva (The World Of Art), out of which sprang Diaghilev's Ballets Russes. Another ancestor through this bloodline was the Italian-born early 19th-century Russian composer Catterino Cavos.

Ustinov was born in Swiss Cottage, London, an almost perfectly spherical 12lb baby and only child, descended as he later said "from generations of rotund men - it was the 214th prize in the lottery of life". He was, according to his family, reading at eight months and displaying his father's gift for mimicry by impersonating Lloyd George at the age of two.

His first drawback was his shape, which led to nicknames at school. He "learned how to survive by emphasising the clumsy and comic aspects of my character", a ploy that became lifelong. His second was his poor maths and science, which prompted him to leave Westminster School without taking exams. But he had already earned his first writing income by selling the London Evening Standard a teasing news story about Ribbentrop's son, a fellow pupil. He was already starting to write plays.

His mother got him into the London Theatre Studio, an acting school run by her friend, the eminent director Michel St Denis. The actor Dirk Bogarde, who shared a dressing room with him, remembered "a rough-haired, scatty boy, blindingly ambitious, streets ahead of anything I'd ever come across ... he scared the shit out of me."

St Denis diagnosed "a dangerous facility which would need counteracting with discipline", a complaint drama critics were to make all Ustinov's life. In 1939, his flair burst out almost overnight in two sketches in a revue at the Players' Theatre in London. One sketch, spoken mainly in invented Swahili, satirised a talk he had heard at school by a cleric on "Christian soldiers in the heart of darkest Africa".

The second was a monologue by Madame Liselotte Beethoven-Fink, an imaginary, Austro-German lieder singer, "grimly mustering the remains of a charm that never existed", as James Agate, the most influential theatre critic of his time, wrote. Agate called it "an immense piece of acting, too macabre and too true to be merely funny".

This was enough to launch him in the theatre. A friend sent Agate a draft of Ustinov's first play, House Of Regrets (1940), a tragicomedy about Russian émigrés not unlike his own family. The critic announced in the Sunday Times, "A new dramatist has arrived." By 1942, just after his 21st birthday, the new dramatist had two plays running in the West End, House Of Secrets and his comedy, Blow Your Trumpet, although the latter flopped.

Simultaneously, thanks to the second world war, Private Ustinov was serving rather insubordinately in the Royal Sussex Regiment. After six awkward months, a transfer to the Army Kinematograph Service enabled him to work as an apprentice scriptwriter on Carol Reed's films The Way Ahead (1944) and The True Glory (1945), and to direct his first film, School For Secrets (1946), about the invention of radar.

In 1944 his play The Banbury Nose, the story of an English upper-class family written backwards in time, led Agate to hail him as "the greatest master of stagecraft now writing in this country... He has as much wit as [Noël] Coward". In 1946 he played the detective, twice his own age, opposite John Gielgud in a legendary stage version of Dostoevsky's Crime And Punishment. Then he directed and starred, as a cowardly Italian conscript, in Private Angelo (1949), a film which won him recognition as a film character actor of flourish, wit and an unusual understated pathos.

In 1950-51 he gave four film performances of this kind: in the French Resistance biopic Odette with Anna Neagle; in the comedy Hotel Sahara; inWe're No Angels as an escaped convict alongside Humphrey Bogart; and as Nero in the Hollywood epic Quo Vadis, which earned him his first Oscar nomination.

But Quo Vadis laid bare an inhibition possibly implanted in his schooldays or by his quarrelling parents; he could not portray passionate feelings without looking foolish.

In retrospect, though, 1952 was his true annus mirabilis. It saw the first night of his most successful play, The Love Of Four Colonels, a cold war satirical burlesque in which Russia, America, Britain and France partition the land in which the Sleeping Beauty lies. Welcomed as a small, if slightly verbose, masterpiece, it ran for years all over the world and became a staple of amateur dramatic societies.

Almost simultaneously, he was diverse and prolific enough to collaborate on BBC radio with Frank Muir, Denis Norden and the actor Peter Jones on the improvised series In All Directions, which is regarded as a forerunner of the Goon Show.

In 1956, he had another durable stage hit with Romanoff and Juliet, another cold-war satire, later filmed. In 1960 came his best film acting as Batiatus, the self-disparaging slave dealer in Stanley Kubrick's film Spartacus. Fellow actors still analyse the almost throwaway technique of understatement with which he upstaged Laurence Olivier during that player's prime and held his own with Charles Laughton, a grand master of underplayed idiosyncracy. It won him an Oscar for best supporting actor - as did Topkapi only four years later.

But his heyday was slipping away. Although he was in 21 more films in the next 30 years, he had no more triumphs; nearly all the films were abysmal. On the stage, the tolerant, discursive, boulevard theatre in which he had flourished was disappearing.

The turning point was the most ambitious project of his life, his effort in 1961 to do justice as film director and producer to Herman Melville's naval allegory of good and evil, Billy Budd. It shot Terence Stamp to stardom but was a near-disaster, partly because to guarantee backing, Ustinov had to miscast himself as the idealistic but impotent authority figure, Captain Vere. He was hamstrung by his awkwardness about passion. The performance was a melodramatic embarrassment, as he acknowledged.

But there was a deeper problem. Ustinov was keen to discuss - as he tried to do in his novels and prose - good, evil and other themes from the old European culture from which he came. In attempting Billy Budd, he felt he had "at last succeeded in fighting my way through tomy own heart".

Yet it began to seem clear that audiences wanted him to be just a funny, foreign fat man. The exotic, clever Europeans he played so well were novel and exciting to the public when such people arrived in Britain and the US as refugees in the war years; by the 1960s they had been absorbed into their host cultures. As a pundit he was handicapped by verbosity. But his talents also straddled too many nationalities for a mass public.

In 1998 he had a critical success in Moscow directing an opera at the Bolshoi. It went unreported in Britain, and in Russia was headlined "Englishman Saves The Bolshoi". He said, "I've never been accused of being English before. It seems rather painful, especially coming from my original country."

Gradually he relaxed - and slightly coarsened - into the role his new admirers seemed to want, into a globetrotting, tax-exiled celebrity who told uproarious tales in funny foreign voices, into the Hercule Poirot film series, which allowed him painfully little range or scope.

He became, as he once said, "a dancing bear", and worked hard at it. Into his late 70s, he travelled arduously for television programmes and gave a spate of good interviews to publicise them. Succeeding the late Orson Welles as a foreign member of the Académie Française, he delivered a tribute to the film director which was full of barely disguised autobiography. He said, "The observer might think that the young man of 25, who had succeeded in leaving the rest of the race so far behind in the first 100 metres, was indeed capable of every victory he could ever wish for. Yet... "

He also said of Welles, "He could have hung up the chains of the dancing bear and given himself over to contemplation and quiet - something he enjoyed but for which he rarely had the peace of mind." His own life would have been sadder if the wellspring of laughter inside him had not run so deep.

But in addition to being a celebrity, he shuttled about on behalf of Unicef, the world children's agency, and was president of the little-known World Federalist Movement. Last year, he was honoured with a graduate college named after him at Durham University, where he was chancellor.

He was married three times: in 1940 to the actress Isolde Denham, with whom he had a daughter, the actress Tamara Ustinov; in 1954 to the actress Suzanne Cloutier, with whom he had a son and two daughters; and in 1972 to the freelance political journalist, Hélène de Lau d'Allemans.